1. Field of the Invention
The present invention relates to a light emitting device using an element (hereinafter referred to as “an organic EL element”) which has an anode layer, a cathode layer, and a film (hereinafter referred to as “organic EL layer”) including an organic compound in which an EL (electro luminescence: luminescence produced by applying an electric field) is produced. As the EL in the organic compound, there are light emission (fluorescence) generated in returning from a singlet excitation state to a ground state and light emission (phosphorescence) generated in returning from a triplet excitation state to a ground state. The present invention particularly relates to a light emitting device in which a metal complex, which is capable of forming pores due to a two dimensional or a three dimensional mesh structure, is applied to a light emitting layer and thus light emitting materials are arranged in the pores to promote light emission of phosphorescence. Note that a light emitting device in this specification indicates an image display device or a light emitting device using an organic EL element as a light emitting element. Also, a module in which a TAB (tape automated bonding) tape or a TCP (tape carrier package) is attached to the organic EL element, a module in which a printed wiring board is provided in the end of the TAB tape or the TCP, and a module in which an IC (integrated circuit) is directly mounted on the organic EL element by a COG (chip on glass) method are all included in the light emitting device.
2. Description of the Related Art
An organic EL element is an element which emits light by applying an electric field thereto. According its light emitting mechanism, a voltage is applied to an organic EL layer sandwiched between electrodes and thus an electron injected from a cathode and a hole injected from an anode are recombined in a light emitting center of the organic EL layer to form a molecule with an excitation state (hereinafter referred to as “a molecular excitation”) and the molecular excitation releases energy to emit light in returning to a ground state.
In the general organic EL element, the organic EL layer is made from a thin film which is thinner than 1 μm. Also, since the organic EL element itself is a self light emission type, a back light as used in a conventional liquid crystal display is not required. Therefore, it is a great advantage that the organic EL element is manufactured to be extremely thin and light weight.
Also, in the case of the organic EL layer with, for example, about 100 to 200 nm, when the carrier mobility of the organic EL layer is considered, a period from the injection of carriers to the recombination is about several tens of nanoseconds. Even if a process from the recombination to the light emission is included in the period, light emission is carried out within an order of microseconds. Therefore, a very high response speed is one of the characteristics.
Further, since the organic EL element is a carrier injection type light emitting element, it can be driven by a direct current voltage and a noise is hard to cause. With respect to a drive voltage, drive is allowed at an order of several volts by a method of selecting an electrode material in which a carrier injection barrier is lowered, a method of introducing a heterostructure (laminate structure), or the like (Reference 1: C. W. Tang and S. A. VanSlyke, “Organic electroluminescent diodes”, Applied Physics Letters, vol. 51, No. 12, 913-915 (1987)). In Reference 1, an alloy of Mg:Ag is used as a cathode and a hetero structure in which a diamine compound and tris(8-quinolinolato) aluminum (hereinafter referred to as “Alq3”) are laminated is used, and thus direct current low voltage drive is realized.
Because of the above characteristics such as a thin type, light weight, fast response, and direct current low voltage drive, the organic EL element attracts an attention as a next generation flat panel display element. Also, the organic EL element is a self light emission type and has a wide view angle. Thus, its visibility is relatively high and it is considered that the organic EL element is effective as an element used for a display screen of mobile equipment.
Here, as described above, the organic EL is a phenomenon in which light is emitted when the molecular exciton is returned to the ground state, and a singlet excitation state (S*) and a triplet excitation state (T*) are allowed as the excitation state of the molecular exciton produced by the organic compound. Also, it is considered that the statistical generation ratio in the organic EL elements is S*:T*=1:3 (Reference 2: Tetsuo Tsutsui: “The Japan Society of Applied Physics, Organic Molecule and Bioelectronics Division, Third Seminar Text”, p.31 (1993)).
However, with respect to a general organic compound, light emission (phosphorescence) from the triplet excitation state (T*) is not observed at a room temperature and only light emission (fluorescence) from the singlet excitation state (S*) is generally observed. This is because the ground state of the organic compound is generally the singlet excitation state (So) and thus a transition T*−So becomes a forbidden transition and a transition S*−So becomes an allowed transition.
That is, only the single excitation state (S*) generally contributes light emission and this is the same in the case of the organic EL element. Thus, it is assumed that a theoretical limitation of internal quantum efficiency (ratio of photons generated to injected carriers) in the organic EL element is 25% based on evidence of S*:T*=1:3.
Also, all generated light is not emitted to the outside and a part of the light cannot be picked up due to refractive indexes which are inherent to organic EL element constituent materials (organic EL layer material and electrode material) and a substrate material. A ratio of the light picked up toward the outside to the generated light is called light pickup efficiency. It is said that the pickup efficiency in the organic EL element which has a glass substrate is about 20%.
From the above reason, even if all the injected carriers form the molecular excitations, it is said that the theoretical limitation of a ratio of photons finally picked up toward the outside to the number of injected carriers (hereinafter referred to as “external quantum efficiency”) is 25%×20%=5%. That is, even if all carriers are recombined, only 5% of the recombined carriers are picked up as light according to calculation.
However, recently, organic EL elements, whish are capable of converting energy released in returning from a triplet excitation state (T*) to a ground state (hereinafter referred to as “triplet excitation energy”) into light to be emitted, are successively reported and their high light emission efficiencies are noted (Reference 3: D. F. O'Brien, M. A. Baldo, M. E. Thompson and S. R. Forrest, “improved energy transfer in electrophorescent devices”, Applied Physics Letters, Vol. 74, No. 3, 442-444 (1999) and Reference 4: Tetsuo Tsutsui, Moon-Jae Yang, Masayuki Yahiro, Kenji Nakamura, Teruichi Watanabe, Taishi Tsuji, Yoshinori Fukuda, Takeo Wakimoto and Satoshi Miyaguchi, “High Quantum Efficiency in Organic Light-Emitting Devices with Iridium-Complex as a Triplet Emissive Center,” Japanese Journal of Applied Physics, Vol. 38, L1502-L1504 (1999)).
In Reference 3, a metal complex with platinum as main metal (hereinafter referred to as “a platinum complex”) is used. Also, in Reference 4, a metal complex with iridium as main metal (hereinafter referred to as “an iridium complex”) is used. Thus, it can be said that it is a characteristic to introduce a third transitions series element as main metal in any metal complex. There is an organic EL element in which the theoretical limitation value of the external quantum efficiency as described above greatly exceeds 5%.
As described in References 3 and 4, with respect to the organic EL element which is capable of converting the triplet excitation energy into light to be emitted, higher external quantum efficiency than a conventional element can be achieved. And, if the external quantum efficiency is increased, a light emission intensity is improved. Thus, it is considered that the organic EL element, which is capable of converting the triplet excitation energy into light to be emitted, has a large share in future developments as a manner for achieving high intensity light emission and high light emission efficiency.
However, since both platinum and iridium are so-called noble metal, the platinum complex and the iridium complex using these metals are expensive and thus it is expected that a cost reduction is hindered in future.
In addition, a color of light which the above iridium complex emits is a green color, that is, a wavelength located in the middle of a visible light region. When the platinum complex is used as dopant, it emits light of a red color with a relatively high color purity. However, there are the following defects. That is, in the case of the platinum complex with a low concentration, a color purity is decreased since a host material also emits light. In the case of a high concentration, light emission efficiency is reduced because of concentration quench. In other words, high efficiency light emission of a red color and a blue color, which have a high color purity, is not obtained from the organic EL element which is capable of converting the triplet excitation energy into light to be emitted.
Therefore, when it is considered that a full color flat panel display is manufactured using light emission colors of red, green, and blue in the future, it is necessary to achieve red color light emission and blue color light emission, which have high external quantum efficiency and a high color purity in the cases of the iridium complex and the platinum complex, using a lower cost material as much as possible.
From such a background, it is desirable that the organic EL element which is capable of converting the triplet excitation energy into light to be emitted, except the organic EL element using the iridium complex or the platinum complex, which already exists, is developed. A most simple method is developing a new organic compound in which light is emitted as phosphorescence at a room temperature. However, a clear molecular design plan is not established until now and is very difficult in many aspects.
Thus, although it is important to develop a new material which emits phosphorescence light, a method of designing a structure of the organic EL layer, in which phosphorescent light emission is promoted, is desirable for light emitting materials used in a conventional organic EL element. This reason is as follows. That is, since various light emission colors have been already obtained in the case of the light emitting materials used in the conventional organic EL element, there is a possibility that various light emission colors are obtained and a large number of low cost materials are present.